1. Field of the Invention
The invention relates to an apparatus and method for measuring meteorological parameters. More particularly, it relates to an apparatus and method for measuring vertical profiles of the concentration of supercooled liquid water.
2. The Prior Art
An intriguing and increasingly more important facet of meteorology is that of weather modification. Of particular interest to meterologists are the processes and techniques surrounding cloud modification so as to induce precipitation. In order to induce precipitation in the atmosphere, nuclei (very small particulates such as dust) must be present on which supercooled liquid water (i.e., water whose temperature is below 0.degree. C. but still exists in the liquid phase) readily condenses and forms an ice crystal. Once the crystal is large enough, it falls as precipitation. Whether this precipitation finally reaches the earth in the form of rain, snow, sleet or hail depends upon air currents, temperature, and humidity.
Many clouds possess the moisture necessary for precipitation, but the temperature of the cloud is too high for ice crystal formation. To induce precipitation, it is common to "seed" the cloud with artificial ice nuclei, such as silver iodide, which have a crystalline structure very much like natural ice. In the presence of silver iodide, the supercooled liquid water molecules behave as if natural ice nuclei were present, thereby catalyzing the formation of precipitation.
It is known in the art that the potential precipitation yield from cloud seeding is related to the supercooled liquid water concentration (hereinafter referred to as "SLW concentration") present in the cloud prior to seeding. Knowledge of SLW concentration is, therefore, an important factor in developing a physical understanding upon which a sound seeding technology can be based and an essential factor in the practical application of weather modification techniques. Moreover, it is believed that SLW concentration measurements may provide significant insights in the study of cloud physics, and may be a sufficiently important indicator of weather patterns as to warrant routine measurement thereof by the U.S. National Weather Service.
Devices, capable of measuring SLW concentrations under certain conditions, are presently found in the prior art. The Johnson-Williams hot wire liquid water content meter, for example, is capable of measuring SLW concentrations at subfreezing temperatures. See C. B. Neal, Jr. and C. P. Steinmetz, "The Calculated and Measured Performance Characteristics of a Heated-Wire Liquid-Water Content Meter for Measuring Icing Severity", NACA Technical Note 2615-52. The Johnson-Williams instrument provides a wire exposed to the atmosphere which makes up one of the resistive elements of a balanced wheatstone bridge. Electronic circuitry monitors the cooling effect on the wire as droplets of supercooled liquid water come into contact with the wire, and the SLW concentration can be derived from this cooling effect.
Another device found in the prior art is the Rosemont Ice Detector manufactured by Rosemont, Inc. of Minneapolis, Minnesota. This instrument was developed in response to a need for a reliable aircraft warning system for the detection of icing on the aircraft. The Rosemont device provides a rigid metallic rod that protrudes from the aircraft and vibrates axially at a very high resonant frequency. Supercooled liquid water accumulates on the rod by contact freezing and decreases the frequency of vibration. When the frequency of vibration decreases to a predetermined level, the device activates heating elements within the aircraft and sounds a warning signal.
Although both the Johnson-Williams and Rosemont devices are capable of measuring SLW concentrations, they were designed for use onboard aircraft and, hence, their applications are somewhat limited to the capabilities of aircraft in gathering meteorological data. For example, it is not possible to measure vertical profiles of the SLW concentration (i.e. the concentration of supercooled liquid water as a function of altitude along an essentially vertical path) in an airplane, nor is it advisable to make measurements in close proximity to mountainous terrain or during periods of severe weather conditions. Such measurements, however, are extremely valuable in providing added insight in the study of cloud physics and weather modification. Moreover, the Johnson-Williams and Rosemont devices are relatively expensive and delicate instruments. The result is that much care must be taken in connection with the operation of these instruments.
A very versatile vehicle for acquiring meteorological information, and one that the National Weather Service has used extensively for years is the weather balloon. Not only is the weather balloon ideally suited for use in measuring vertical profiles, it can be used for collecting data over any type of terrain, under any weather conditions. Each year thousands of instrumented weather balloons are released and monitored to gather meteorological information.
It is common for such balloons to carry with them an electronic device called a radiosonde which combines meterorological sensing equipment with a radio transmitter. As a radiosonde equipped balloon ascends into the atmosphere, typically at a rate of approximately 1000 feet per minute, meteorological equipment measures pressure, temperature, and humidity (by use of a barometer, thermistor, and hygrometer, respectively). These data are transmitted to a ground-based receiving station and are recorded. Winds at different levels can be charted from the data obtained from the meteorological sensing equipment and from tracking the balloon.
Since the balloons typically ascend into the atmosphere until they burst, it is not always possible to recover the electronics carried by the balloons. Thus, because of prohibitive costs, it is not economically feasible to simply combine either the Johnson-Williams or the Rosemont device with a standard radiosonde. Indeed, an inexpensive and expendable device for airborne measurements of SLW concentrations is not found in the prior art.
It would, therefore, be an advancement in the art to provide a device for measuring SLW concentrations that is (1) readily integrated into a standard radiosonde or other similar device, (2) inexpensive and therefore expendable, and (3) accurate and reliable in its operation.